Ultrasound beamformer-based channel data compression

ABSTRACT

Ultrasound beamformer-based channel data compression allows for software-based image formation. To increase the amount of data transferred, ultrasound beamformer-based channel data compression is provided. A beamformer is used to compress instead of or in addition to traditional beamformation. The compression reduces the data bandwidth while allowing reconstruction of the original channel data.

BACKGROUND

The present embodiments relate to medical diagnostic ultrasound imaging.In particular, channel data compression is provided for ultrasoundimaging using software.

As computer processing power increases, ultrasound systems have beengradually transferring hardware signal processing functionality intosoftware. Software-based signal processing is advantageous for flexibledevelopment, better maintainability, cost, and quick experimentation.The primary problem facing a software-based ultrasound system is gettinglarge amounts of single-element, raw channel data from an ultrasoundfront-end into a computer memory for processing. When the system channelcount is high (e.g., 128 elements or channels), transfer data rates maybe extremely high and challenge the maximum data rates of even the moststate of the art computer bus. To deal with such large transfer rates,additional or custom hardware may be used, but this defeats the primarypurpose of hardware reduction. The maximum acoustic frame rate and/orthe number of channels may be reduced, but this results in decreasedimage quality.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, computer readable media, instructions, and systems forultrasound imaging. To increase the amount of data transferred,ultrasound beamformer-based channel data compression is provided. Abeamformer is used to compress instead of or in addition to traditionalbeamformation. The compression reduces the data bandwidth while allowingreconstruction of the original channel data.

In a first aspect, a system is provided for ultrasound imaging. Atransmit beamformer is configured to transmit first beams for a firstimaging mode and second beams for a second imaging mode. A transducerincludes elements for receiving acoustic echoes in response to the firstand second beams. A receive beamformer is configured to receiveelectrical signals from the elements, to beamform samples from theelectrical signals responsive to the first beams, and to compress theelectrical signals responsive to the second beams using a Fouriertransform applied across the elements for each time. A processor isconfigured to generate imaging information for the first imaging modefrom beamformed samples and to generate imaging information for thesecond imaging mode from the compressed electrical signals.

In a second aspect, a method is provided for ultrasound beamformer-basedchannel data compression. Channel data is received from elements of atransducer array. The channel data is encoded laterally across the arraywith a delay and sum beamformer with a set of basis functions. The basisfunctions reduce the amount of data to send over a computer bus andenable recovery of the channel data from the output of a decodingoperation. The basis set encoding is repeated for new frames of channeldata and the output of the encoding is transmitted over the computerbus.

In a third aspect, a method is provided for ultrasound beamformer-basedchannel data compression. A plurality of channels of element signals issampled. A receive beamformer transforms samples from the samplingdomain into frequency data in a spatial frequency domain. The frequencydata is inverse transformed. A processor generates an ultrasound imagefrom an output of the inverse transforming.

Further aspects and advantages of the invention are discussed below inconjunction with the preferred embodiments. The present invention isdefined by the following claims, and nothing in this section should betaken as a limitation on those claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the Figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram of one embodiment of a system for ultrasoundimaging;

FIG. 2 is a block diagram of one embodiment of a receive beamformer forultrasound beamformer-based channel data compression;

FIG. 3 is an example graph of compression ratio for Fouriertransformation by a beamformer as a function of steering angle andfrequency;

FIG. 4 is an example process for frequency domain beamforming withFourier encoded channel data from a beamformer; and

FIG. 5 is a flow chart diagram of one embodiment of a method forultrasound beamformer-based channel data compression.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Ultrasound raw channel data is compressed for transmission to aprocessor or system for ultrasound imaging. Any basis function set, suchas a Fourier basis, may be used. The compression is achieved in thelateral dimension or across the array, allowing use of a conventionaldelay-and-sum beamformer. When the number of frequency domain ‘beams’ oramount of frequency domain data representing the channel data is lessthan the number of channels, an effective compression is achievedcompared to directly transferring raw channel data. Raw channel datacompression of 3:1 or more may be provided. Commonly used hardware(i.e., a delay-and-sum beamformer) performs lateral encoding of the rawchannel data to reduce the total amount of raw channel data sent overthe PCIExpress or other bus to a computer.

This re-use of existing hardware may keep development time and costs lowand/or allow the use of new beamforming technologies, such as frequencydomain beamforming applied in software. For Fourier beamforming, thebeamformer-created Fourier data may be Fourier transformed over time ordepth, providing a two-dimensional transform of the channel data.Beamformation may then be provided in the two-dimensional Fourierdomain.

In one embodiment, multiple channels of raw data from a transmissionevent are sampled and sent to a hardware delay-and-sum beamformer. Thebeamformer applies a combination of complex apodization weights and/orlateral delay profiles that allow the beamformer to form ‘beams’ in thefrequency space of the raw channel data, effectively Fourier encodingthe data. Other apodization and/or delay profiles may be used toimplement other basis functions that substantially enable recovery ofchannel information. Lossless or lossy recovery of the channel data maybe used or the data may be further transformed for beamforming from adomain other than the spatial-temporal domain of the channel data.

Since a beamformer is used for compression, the same beamformer may beused for conventional delay-and-sum beamformation. The beamformeroperates “on the fly” to switch between encoding for compression (i.e.,channel basis function encoding) and conventional beamformation. Forexample, data from one imaging mode (e.g., B-mode) is Fourier encodedand compressed for high-quality, low-data rate imaging to be providedwhile data from another imaging mode (e.g., flow or color mode) isbeamformed conventionally for typical image processing. Alternatively,in other modes, the beamformer is bypassed completely, allowinguncompressed raw channel data to be collected and processed entirelyusing software-based beamformation.

FIG. 1 shows one embodiment of a system for ultrasound imaging. Thesystem performs the method described below for FIG. 4 or 5 or adifferent method. The system operates using both conventionalbeamformation and compression of channel data by the receive beamformer56. In other embodiments, the system only operates using the compressionof channel data by the receive beamformer 56. Alternatively, the systemperforms any beamforming with the processor 62 and uses the receivebeamformer 56 for compression.

The ultrasound system includes a transmit beamformer 52, a transducer54, a receive beamformer 56, a display 60, a processor 62, and a memory64. Other systems may be used. Additional, different or fewer componentsmay be provided. For example, a detector and/or scan converter areprovided. As another example, a user input device (e.g., mouse and/orkeyboard) is provided for accepting user selection of an imagingapplication (e.g., cardiac imaging) and/or other configuration, such asa selection of imaging parameters. In yet another example, the transmitbeamformer 52, transducer 54, and/or display 60 are not provided, suchas where raw channel data is provided from any source for compressionand transfer to the processor 62 with or without more processing.

The system is a medical diagnostic ultrasound imaging system. Imagingincludes two-dimensional, three-dimensional, B-mode, Doppler, colorflow, spectral Doppler, M-mode, strain, elasticity, harmonic, contrast,or other imaging modalities now known or later developed. The ultrasoundsystem is a full size cart mounted system, a smaller portable system, ahand-held system, or other now known or later developed ultrasoundimaging system. In another embodiment, the processor 62 and memory 64are part of a separate system. For example, the processor 62 and thememory 64 are a workstation or personal computer operating independentlyof or connected with the beamformers 52, 56.

The transmit beamformer 52 is one or more waveform generators,amplifiers, delays, phase rotators, multipliers, summers,digital-to-analog converters, filters, combinations thereof, and othernow known or later developed transmit beamformer components. Thetransmit beamformer 52 is configured into a plurality of channels forgenerating transmit signals for each element of a transmit aperture. Thetransmit signals for each element are delayed and apodized relative toeach other for focusing acoustic energy along one or more scan lines.Delay is implemented as a temporal delay of a generated waveform, delayof generating a waveform, and/or phase shift of a waveform in generatingor after being generated. Signals of the same or different amplitudes,frequencies, bandwidths, delays, spectral energy distributions or othercharacteristics are generated for one or more elements 70 of thetransducer 54 during a transmit event.

The transmit beamformer 52 is configured to generate any number ofbeams. One or more beams may be generated at a same time. Each beam isfocused along a transmit scan line to allow reception of a receive beamalong the same scan line. A sequence of beams steered in any format(e.g., linear, Vector®, or sector) may be generated. A broad beam may begenerated for receiving along a plurality of receive scan lines inresponse to the single transmit broad beam. Plane wave, diverging wave,or infinite focus may be used for the broad beam.

In one embodiment, the transmit beams are generated for each of multipledifferent imaging modes. For example, color, flow or Doppler-modeimaging and corresponding transmit beams are interleaved with B-modeimaging and corresponding transmit beams. Beam, group of beam, or frameinterleaving may be used. During on-going or continuous scanning of apatient, the transmit beams for the different imaging modes aretransmitted in an interleaved sequence. One or more beams for oneimaging mode may also be used for the other imaging mode.

The transducer 54 is a one-dimensional, multi-dimensional, or other nowknown or later developed array of elements 70. Each element 70 of thetransducer 54 is a piezoelectric, microelectromechanical, capacitivemembrane ultrasound transducer, or other now known or later developedtransduction element 70 for converting between acoustic and electricalenergy. Each of the transducer elements 70 connect to the beamformers52, 56 for receiving electrical energy from the transmit beamformer 52and providing electrical energy responsive to acoustic echoes to thereceive beamformer 56. In response to transmission of the transmitbeams, acoustic echoes are received by the elements 70 and convertedinto electrical signals. These electrical signals from any of theelements 70 in the receive aperture are passed separately to the receivebeamformer 56.

The receive beamformer 56 is configured to acquire ultrasound datarepresenting a region of a patient. Electrical signals representing theacoustic echoes from a transmit event are passed to the channels of thereceive beamformer 56. In one embodiment, the receive beamformer 56 isformed from one or more application specific integrated circuits,processors, controllers, or other integrated circuits. The receivebeamformer 56 includes a plurality of channels for separately processingsignals received from different elements of the transducer 54. Eachchannel may include delays, phase rotators, amplifiers, filters,multipliers, summers, analog-to-digital converters, control processors,combinations thereof, or other now known or later developed receivebeamformer components. The receive beamformer 56 also includes one ormore summers for combining signals from different channels into abeamformed signal. A subsequent filter may also be provided. Other nowknown or later developed receive beamformers may be used.

FIG. 2 shows one embodiment of the receive beamformer 56 connected withelements 70 of an array. Each channel of the receive beamformer 56connects with one element 70, but may connect with more than one element70 and/or different elements 70 through a multiplexer. Any number ofchannels is provided. Each channel includes an amplifier 72 and a phaserotator 74. Other amplification devices or circuits may be used. Thedelay for the delay-and-sum beamformer 56 may be implemented with otherdevices instead of or in addition to the phase rotator, such as a delayor buffer. The phase rotators 74 may be positioned prior to or after theamplifiers 72.

The amplifiers 72 are programmable to apply a desired amount ofamplification. An apodization profile 78 is provided by a processor,memory, or controller. The apodization profile 78 provides for the sameor different amplification by the different amplifiers 72. Relativeweighting of one channel to the others is provided by the apodizationprofile 78.

The phase rotators 74 are programmable to apply a desired amount ofphase shifting or other delay. A delay profile 80 is provided by aprocessor, memory, or controller. The delay profile 80 provides for thesame or different phase shift by the different phase rotators 74.Relative phase rotation of one channel to the others is provided by thedelay profile 80.

The summer 76 sums information from the channels in a given receiveaperture. The summer 76 is a digital summer, but may be an analogsummer. A single summer is provided to sum outputs from all of thechannels. Alternatively, hierarchal summers or cascade of summers areused. At a given time or clock cycle, each channel outputs data, such asdigital samples. The output data represents lateral position along a oneor multi-dimensional array for the given time or clock cycle. The outputdata is relatively delayed and apodized based on the delay profile 80and apodization profile 78. The summer 76 sums the outputs from thechannels at each of a sequence of times.

Referring to FIGS. 1 and 2, the receive beamformer 56 receiveselectrical signals from the elements 70. The electrical signals aredigitized or sampled. These samples are provided as channel data foreach channel from respected elements 70. These samples are eitherbeamformed or compressed. For example, samples for one mode of imagingare beamformed using conventional delay-and-sum beamformation based onthe geometry of the elements 70 relative to the location in the patientfor which the echo response is sampled. Samples for another mode ofimaging are compressed. Rather than using the geometry of elements 70 tosample location, the apodization and/or delay profiles 78, 80 are basedon basis function encoding for compression. A channel encoding operationis performed by the beamformer 56.

The switching between the modes is dynamic or on the fly. The switchingmay occur based on imaging conditions and imaging requirements (e.g.,achievable compression ratio) and/or to optimize system performance(e.g., reduce the computational load). In one example, a B-mode/Colorframe sequence is run on the ultrasound imaging system. The B-mode imageis formed using broad beam transmissions (e.g. plane waves) toaccomplish high-frame rate imaging, while conventional focused beamtransmissions are used for the Color data to enhance imagingsignal-to-noise ratio. In this example, B-mode images are formed usingfrequency domain beamforming, so it is more computationally efficient toFourier encode the raw channel data using the beamformer 56 operating inthe channel-encoding mode. While in the case of the color frame data,the beamformer forms image lines close to the areas where the focusedtransmissions are occurring. For these frames, the beamformer isoperates in the conventional delay-and-sum operation. Other combinationsof different modes may be used. The color data may be encoded, and theB-mode data may be conventionally beamformed. More than two modes ofimaging may be used at a time.

For conventional beamforming, the receive beamformer 56 outputs in-phaseand quadrature, radio frequency or other data representing one or morelocations in a scanned region. The channel data is relatively weightedand delayed based on geometric relationship of element location tosample location in the patient. This weighting aligns the data output bythe channels so that the data represents the same location in thepatient. By summing the data, a beamformed sample representing thereceived echo from that location is formed. By repeating the processover time, a set of samples representing a beam along a scan line isformed. The receive beamformer 56 operates in the traditionaldelay-and-sum beamforming operation (i.e. generating image “lines”).

For compression, the electrical signals responsive to the transmit beamor beams are processed by the receive beamformer 56. Instead of usearray geometry and beamform focus location in the patient for theapodization and delay profiles 78, 80, a basis function for compressionor encoding the channel data is applied with the profiles 78, 80. Ratherthan delaying the data so that the output to the summer 76 from eachchannel corresponds to echoes from the same location in the patient, thedelays and apodization implement a basis function for compression orencode the data in a recoverable way.

Any complete basis function set may be used, such as a wavelet basis ora Fourier basis. The chosen basis functions transform the channel datainto a different domain in which less data may represent the same orsubstantially same information. Substantially is used to account forlossy compression. By using basis function encoding, the channel datamay be recovered, such as by applying an inverse transform operation.

In one embodiment, the receive beamformer 56 uses a Fourier transformapplied across the elements for each time. At a given clock cycle, eachchannel is processing data received by the elements at a same time.Alternatively, relative delay may have been applied. The Fouriertransform is applied to the channel data across channels or laterally.The Fourier transform spatially encodes the channel data betweenchannels rather than over time (i.e., along depth).

The delay-and-sum beamforming operation is represented in a general formas:

b(t) = ∫_(x)s(t − τ(x), x)a(x) x

where t is time, x is element position, s(t) is the raw channel data,τ(x) is the lateral-dependent delay profile, a(x) is the lateralapodization profile, and b(t) is the beamformed signal. In order toachieve lateral Fourier compression of the data using conventionaldelay-and-sum beamforming, the lateral-dependent delay profile and/orlateral apodization profile implement the basis function encoding forcompression. The phase rotators or other delay elements and amplifierswith the summer implement the encoding. The resulting summation outputby the beamformer is compressed channel data.

In one approach, the phase or delay profile for the geometricrelationship is zero. No relative shift between elements is implementedbased on the different distances or other geometrical relationship ofthe elements to the location in the patient. The apodization function isa complex apodization function. To implement the complex apodization,some phase shift may be applied by phase rotators or other delayelements. The only delay profile used is to implement the complexapodization function. Alternatively, a shift based on geometricalrelationship is included, but an additional shift is provided forcompression using the complex apodization profile.

In this approach, the beamforming profiles 78 and 80 provide for complexapodization. The delay profile of τ(x)=0 is used, letting a(x)=e^(j2πf)^(x) ^(x). This results in a Fourier transform across channels. Theencoding is repeated for each time. For a two-dimensional Fouriertransform, a further processor may transform along the temporal or depthdirection, yielding:

B(f)=∫_(x) S(f,x)e ^(−j2πf) ^(x) ^(x) dx=f _(x)).

The spectrum of the beamformed or compressed data is equal to a singleCartesian line (along f_(x)) in the two-dimensional spectrum of the rawchannel data. In other embodiments, the Fourier transform is not appliedin the temporal or depth direction. Instead, the compressed data isprovided to a processor and inverse compression is applied toreconstruct the channel data.

In another approach, a unity apodization function is applied by theamplifiers. The phase rotators or delays provide or add a linear delayprofile for implementing compression laterally across the channels andcorresponding elements 70. The linear delay profile is a function of thelateral position of the elements 70 and the steering angle of thereceive beam and/or transmit beam. In this radial Fourier samplingapproach, the beamformer apodization is chosen to be a(x)=1 and a lineardelay profile of:

${\tau (x)} = \frac{x\mspace{11mu} \tan \mspace{11mu} \theta_{s}}{c}$

is used where θ_(s) is the steering angle for the linear delay profile.As discussed above for the first approach, the Fourier transform isapplied across the array. A further Fourier transform may later beapplied, such as by the processor 62. Taking a Fourier transform in t(time or axially) gives:

${B(f)} = {{\int_{x}{{S\left( {f,x} \right)}^{{- {j2\pi}}\; f\frac{\tan \mspace{11mu} \theta_{s}}{c}x}\ {x}}} = {{S\left( {f,{f\frac{\tan \mspace{11mu} \theta_{s}}{c}}} \right)}.}}$

The one-dimensional beamformed spectrum maps directly onto a radial linein the two-dimensional spectrum of the raw channel data. The slope ofthe line is directly related to the received steering angle, but is nota 1:1 mapping as seen in the conventional Fourier-slice theorem.

Other approaches may be used. For example, the Fourier transform orother transform may use both the apodization and delay profile toimplement the basis function encoding. The summation by the summer 76completes the data encoding and compression. After applying theapodization and delay, the summation finishes the implementation of thebasis function encoding.

The compression with eventual beamformation by the processor 62 is usedfor all ultrasound imaging by the system or only for some of theultrasound imaging. In one embodiment, the imaging parameters and/orconditions are used to determine whether a given imaging mode usescompression or conventional beamformation. For example, the imaging modewith a higher imaging frequency, a greater field of view, greater framerate, or other characteristic uses compression and other imaging modsuse conventional beamformation by the beamformer. For a given mode, thecompression ratio may be used to select between compression fortransmission of data for software beamformation and no compression fortransmission of the channel data for software beamformation.

The compression ratio indicates the effectiveness of compression ascompared to passing channel data to a processor 62 for beamformation.Beamformation reduces the amount of data, but limits the usefulness ofthe data. By providing channel data to the processor 62, software-basedimage formation may be used. The compression ratio of uncompressedchannel data to compressed channel data may be used to select which formof channel data to pass on.

The compression ratio is, for example, represented as:

${CR} = \frac{{uncompressed}\mspace{14mu} {size}}{{compressed}{\mspace{11mu} \;}{size}}$

The Fourier compression is influenced primarily by the imagingparameters and/or conditions. The potential compression ratios may becalculated as a function of the main imaging parameters, such as f#,imaging frequency, acceptance angle, steering angle, aperture size, orothers. When imaging with plane wave transmissions with a transmissionangle of θ_(T)=0° and using a transducer of length L, the maximumlateral spatial frequency is given as:

$f_{xh} = \frac{\sin \mspace{11mu} \theta}{\lambda}$

where θ is the reception angle and A is the wavelength. Using thisexpression plus the lateral frequency spacing of:

${{\Delta \; f} = \frac{1}{L}},$

the number of frequency lines needed for Nyquist sampling is given as:

$N_{lines} = {\frac{2\; f_{x}}{\Delta \; f} = {\frac{2L\mspace{11mu} \sin \mspace{11mu} \theta}{\lambda}.}}$

The compression ratio is then calculated as:

${{CR} = {\frac{N_{channels}}{N_{lines}} = \frac{\lambda \; N_{channels}}{2L\mspace{11mu} \sin \mspace{11mu} \theta}}},$

showing that the compression is dependent upon the imaging frequency,1/λ, and also upon the selected transducer acceptance angle, θ.

As an example, FIG. 3 shows compression ratio as a function of theacceptance angle of the transducer when using typical transducerparameters for a mid/high frequency linear transducer (e.g. 192 elementtransducer array with a 200 μm element pitch and center frequenciesranging from 4-8 MHz). The compression ratio as a function of acceptanceangle is given for three different imaging frequencies. These plotsdemonstrate that raw channel data may be laterally Fourier compressed ascompared with directly transferring raw channel data with compressionratios of greater than one, such as 2-3 (or higher). The amount ofcompression is driven by selected imaging optimization parameters. Wherethe ratio is close to or less than 1, then Fourier encoding is not asefficient as simply sending the raw channel data directly to thecomputer. For compression ratios larger than 1, the Fourier encodingwith the beamformer effectively compresses the data and reduces theamount of data to be sent. For example, a transducer with a 30-degreeacceptance angle that operates with a center frequency of 4 MHz is to beused. The compression ratio is close to 2:1, so beamformer Fouriercompression in this situation would be optimal.

Where compression is used (e.g., Fourier encoded), the compressedchannel data is transferred to the processor 62 and/or the memory 64.For example, the transfer is to a computer using a computer bus, such asa PCIExpress bus. The processor 62 of the computer uses software toinverse the Fourier transform to reconstruct the channel data to thenperform delay-and-sum beamforming using software or applies temporalFourier transformation for Fourier based beamformation using software.The compressed channel data may be used for any process by the processor62.

The processor 62 is a control processor, filter, general processor,application specific integrated circuit, field programmable gate array,digital components, analog components, hardware circuit, combinationsthereof and other now known or later developed devices for imageprocessing to enhance an image. The processor 62 is configured, withcomputer code, firmware, and/or hardware, to generate ultrasound imageinformation from the compressed and/or never compressed channel data.

In one embodiment, the processor 62 is part of a computer forimplementing any ultrasound imaging using software. Beamformation,detection, scan conversion, mapping to display values, temporalfiltering, spatial filtering, image enhancement, graphics generation,combinations thereof, and/or other ultrasound imaging process areperformed by the processor 62. In alternative embodiments, separatecomponents (e.g., beamformer, detector, filter, and/or scan converter)are provided and operate with or as part of the processor 62 forultrasound imaging. The generated image information is ultrasoundinformation representing the patient at any stage of processing (e.g.,beamformed, detected, scan converted, and/or mapped to display values).

For dual-mode operation, the processor 62 is configured to generateimage information in one or more imaging modes from channel data and/orbeamformed data. The processor 62 may control the receive beamformer 56to select between bypass, delay-and-sum beamformation, and/orcompression. Where the receive beamformer 56 operates for conventionalreceive beamformation, the processor 62 receives the beamformed data andgenerates the image information. Where the processor 62 instead receiveschannel data without compression (e.g., beamformer 56 is bypassed), theprocessor 62 performs the beamforming from the channel data to generatethe imaging information.

Where the receive beamformer 56 compresses the channel data, theprocessor 62 uses the compressed information to generate the imaginginformation. For example, electrical signals for one imaging mode arecompressed by the beamformer. The processor 62 uses those compressedsignals to generate imaging information.

In one embodiment, the compressed channel data is used for delay-and-sumbeamformation. The compressed channel data is decompressed toreconstruct the channel data. An inverse Fourier transform is applied tothe compressed electrical signals to reconstruct in a lossy or losslessmanner the electrical signals from the elements. Using a set ofreconstructed electrical signals from the different elements 70 overtime, the processor 62 then applies a delay profile and an apodizationprofile to the reconstructed channel data and sums the results. Phasingor absolute delay may be used to implement the delay profile. Oncebeamformed, the processor 62 image processes the beamformed data, suchas detecting, scan converting, and/or display mapping.

In another embodiment, the compressed channel data is used for othertypes of beamformation or image formation. Rather than use delay-and-sumbeamformation, the echo return from different locations is derived usinganother process, such as a process in the domain of the basis functionused to compress. For example, another Fourier transform is applied as afunction of time or depth to the compressed channel data. Since theFourier-based compression is lateral, the axial transform applied by theprocessor 62 results in a two-dimensional transformation. Thetwo-dimensional transformed channel data in the frequency domain is usedfor beam or image formation, such as disclosed in U.S. Pat. No.6,685,641.

FIG. 4 shows one representation of beamformation in the frequencydomain. The compressed or Fourier encoded channel data is furtherFourier transformed in time. The processor 62 multiplies the channeldata in the frequency domain with a focusing function and theninterpolates in the frequency domain from the results of themultiplication. Any of various interpolations may be used, such as aStolt interpolation. The encoded data maps directly into the k-space ofthe beamformed image using conventional Stolt interpolation mappings.Where the encoding uses a complex apodization, the Stolt interpolationmapping is given by:

f_(x)^(′) = f_(x)$f_{z}^{\prime} = {\frac{f}{c}{\left( {1 + \sqrt{1 - {\lambda^{2}f_{x}^{2}}}} \right).}}$

Where the encoding uses a linear delay profile, a modified Stoltinterpolation is used, as represented by:

$f_{x}^{\prime} = \frac{f\mspace{11mu} \tan \mspace{11mu} \theta_{s}}{c}$$f_{z}^{\prime} = {\frac{f}{c}{\left( {1 + \sqrt{1 - {\tan^{2}\theta_{s}}}} \right).}}$

After the interpolation, the processor 62 generates the imaginginformation by applying an inverse Fourier transformation. An inversetwo-dimensional transformation is applied, resulting in beamformedvalues for each of different locations in the patient.

Other Fourier beamforming may be used. Since the data is in thefrequency domain, any other k-space image formation or filtering may beapplied. For example, magnetic resonance or computed tomography imageformation or image filtering techniques using k-space data may beapplied by the processor 62.

The processor 62 uses the imaging information to generate one or moreultrasound images. For example, B-mode images are generated. M-mode,color or flow mode, Doppler or spectral mode, or other modes of imagingmay be used. In one embodiment using dual-modes of imaging, imaging fromthe different modes are combined into one image, such as an overlay ofcolor flow on B-mode. The imaging information used for one mode isderived from beamformer-compressed channel data, and the imaginginformation used for another mode is derived from beamformed data orchannel data received without compression by the processor 62. Both orall modes of imaging may be derived from beamformer-compressed channeldata in other embodiments.

The display 60 is a monitor, LCD, LED, plasma, projector, printer, orother now known or later developed display device. The processor 62generates display signals for the display 60. The display signals, suchas RGB values, are output by the processor 62 to the display 60 or to adisplay buffer. The display 60 is configured to display an imagerepresenting the scanned region of the patient, such as a B-mode image.The image represents the scan region. The image is generated from atleast some data that was at one point compressed by a delay-and-sumbeamformer.

The memory 64 is a computer readable storage medium having storedtherein data representing instructions executable by the programmedprocessor for ultrasound imaging. The instructions for implementing theprocesses, methods and/or techniques discussed herein are provided oncomputer-readable storage media or memories, such as a cache, buffer,RAM, removable media, hard drive or other computer readable storagemedia. Computer readable storage media include various types of volatileand nonvolatile storage media. The functions, acts, or tasks illustratedin the figures or described herein are executed in response to one ormore sets of instructions stored in or on computer readable storagemedia. The functions, acts or tasks are independent of the particulartype of instructions set, storage media, processor or processingstrategy and may be performed by software, hardware, integratedcircuits, firmware, micro code and the like, operating alone or incombination. Likewise, processing strategies may includemultiprocessing, multitasking, parallel processing and the like. In oneembodiment, the instructions are stored on a removable media device forreading by local or remote systems. In other embodiments, theinstructions are stored in a remote location for transfer through acomputer network or over telephone lines. In yet other embodiments, theinstructions are stored within a given computer, CPU, GPU or system.

The memory 64 alternatively or additionally stores channel data,compressed channel data, reconstructed channel data, or other ultrasounddata from any stage of processing. For example, the memory 64 receivesthe beamformer-compressed channel data from the receive beamformer overa bus and stores the data for access by the processor 62.

FIG. 5 shows one embodiment of a method for ultrasound beamformer-basedchannel data compression. Rather than using dedicated compressionhardware for each receive channel, a receive beamformer may be used tolaterally compress. The embodiment of FIG. 5 is directed to compressingchannel data with a beamformer for subsequent beamformation aftertransfer of the compressed channel data and alternatively performingbeamformation on channel data that is not compressed. In otherembodiments, dual-mode operation is not provided, so just compressionwith subsequent image formation is performed.

Additional, different, or fewer acts may be provided. For example, act18 is not provided. As another example, act 20 is not provided. Acts forother imaging operations, such as detection, filtering, scan conversion,and display mapping, may be performed.

The acts are performed in the order shown or a different order. Forexample, act 18 is interleaved with acts 14 and 16. Act 18 may beperformed before, after, or in parallel (simultaneously) with acts 14and/or 16.

In act 12, a beamformer receives channel data from elements of atransducer array. An ultrasound system acquires ultrasound data from ascan of tissue, blood, or other part of a patient. The ultrasound datarepresents the patient. A medical diagnostic ultrasound system applieselectrical signals to a transducer, which then converts the electricalenergy to acoustic energy for scanning a region of the patient.Waveforms at ultrasound frequencies are transmitted. Echoes are receivedand converted into electrical signals by elements of the transducer. Anytype of scan, scan format, or imaging mode may be used. For example,harmonic imaging is used with or without added contrast agents. Asanother example, B-mode, color flow mode, spectral Doppler mode, M-mode,Elastography or other imaging mode is used.

Each element or group of elements outputs signals in a respectivechannel. Any number of channels is provided, such as 32, 64, 128, 256,or other number. The elements of the receive aperture each output to aseparate or independent channel.

The channel signals may be amplified, such as with depth gaincompensation. The channel signals may be converted from analog signalsto digital samples. An analog-to-digital converter of each channelsamples the element signals, creating channel data. The signals orsamples may be filtered and/or buffered. Additional, different, or fewerprocesses may be applied, such as no conversion to digital for an analogbeamformer.

In act 14, the receive beamformer encodes the channel data. Using thedelay and apodization functions of each channel of a delay-and-sumbeamformer, the channel data is encoded laterally across the array. Eachchannel contributes a sample to the encoding. The sample of each channelis delayed (e.g., delayed or phased) relative to other samples andapodized (e.g., changed in amplitude) relative to other samples for thattime. Rather than or in addition to delay and apodization due togeometry of the element and location of interest in the patient for thattime, the apodizations and delays applied to the samples of thedifferent channels are based on a basis function for compression. Theapodized and delayed samples from each channel are summed, providing forthe lateral or across channel encoding.

Any appropriate basis function set may be used for encoding. Forexample, a wavelet or Fourier basis set is used. The basis functionsprovide for lossless or lossy recovery of the channel data.Delay-and-sum beamformation performed conventionally is not reversible.The channel data cannot be recovered. By encoding with basis functions,the channel data may be recovered or substantially recovered from thesummed output of the beamformer. Substantially accounts for lossycompression, where 80% or more of the channel data may be recovered. Theamount of compression loss is dependent on the number of basis functionsused to encode the data. In the example of Fourier compression, the lossmanifests itself in the form of poorer lateral image resolution. Byapplying an inverse transform or other basis function related inverseprocess, the channel data may be reconstructed, if desired.

In one embodiment, the beamformer implements a Fourier transform byencoding. The receive beamformer transforms samples from the lateralsampling domain of the channels into frequency data in the spatialfrequency domain. For example, the encoding uses an apodization profilebeing unity and a linear delay profile. As another example, the encodinguses a geometry-based delay profile being zero and a complex apodizationprofile. The apodization and/or delay profiles and subsequent summationprovide the encoding in the frequency domain.

The encoding act 12 may be repeated. The encoding is lateral across thearray or channels for a given time. Echoes are received by each elementover time. The encoding is performed for different times, encodingadditional channel data as the channel data is received. The result is aset of laterally transformed data output over time by the receivebeamformer.

In act 16, the encoded channel data is transmitted. The beamformeroutputs the compressed data to a buffer, memory, interface, orcommunications path. The output is provided to a bus, cable, or othertransmission line to a computer, processor, or memory. Any transmissionformat or packaging may be used, such as transmitting on a PCIExpressbus.

As the receive beamformer laterally encodes, the resulting data istransmitted. The transmission occurs upon each repetition over time.Alternatively, output data from one or more times is buffered andpacketized with output data from a different time.

For software-based image processing, the computer receives transmissionof the encoded channel data. The computer then performs any imageprocessing. In one embodiment, the computer applies an inversetransform, such as an inverse Fourier transform, to recover the channeldata. Software-based beamforming is performed by the processor using therecovered channel data. In another embodiment, the processor of thecomputer performs Fourier beamforming. Instead of recovering the channeldata, a further Fourier transform is applied as a function of time.Multiplication with a focusing function and interpolation in frequencyspace are performed. The computer performs a two-dimensional, spatialinverse Fourier transform to determine the intensity or echo return foreach of different locations in the patient distributed in one, two, orthree dimensions. In other embodiments, different processes are appliedby the computer to the compressed channel data.

The processor inverse transforms the frequency or other encoded datainto object or real space. The processor generates an ultrasound imagefrom an output of the inverse transformation. Any image processing maybe used, such as beamformation, detection, estimation, filtering, scanconversion, and/or display mapping. The channel data or beamformed datais used to create an ultrasound image representing the scanned patient.

Any of the software processes applied by the processor may be easilychanged without requiring a hardware change. The use of a beamformer forcompression allows for use of a common component in ultrasound tocompress data for transfer into or access by the computer andcorresponding software-based image processing.

In act 18, the beamformer is provided for a dual-mode of operation. Forsome channel data, such as for some imaging modes, parameter settings,and/or imaging conditions, the beamformer applies delay-and-sumbeamformation in object or real space. Instead of compressing,conventional beamformation is applied. The resulting beamformed data istransmitted to the computer or other image processor for generating animage. Alternatively, the beamformer is not used and the channel data istransmitted to the computer. The computer applies the beamformationusing a software-based approach and a general processor.

Where dual-mode operation is provided, the compression by the beamformerand the beamforming or bypass of the beamformer by the channel data areinterleaved. Channel data responsive to some transmissions iscompressed, and channel data responsive to other transmissions is notcompressed. The compression or no compression for an imaging session areinterleaved by line, groups of line, or scan of a field of view basis.

In act 20, an image is generated. The processor or ultrasound systemgenerates the image from the ultrasound data. The inverse transformeddata and/or channel data not subjected to compression are used togenerate an image on a display.

The generated image is a B-mode, color flow mode, M-mode, pulsed waveDoppler, contrast agent, harmonic, other ultrasound image, orcombination thereof. The image represents the patient at a given time orover time. The image may represent one or more sample locations withinthe patient, such as a planar or volume region.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

I (We) claim:
 1. A system for ultrasound imaging, the system comprising:a transmit beamformer configured to transmit first acoustic energy for afirst imaging mode and second acoustic energy for a second imaging mode;a transducer comprising elements for receiving acoustic echoes inresponse to the transmission of the first and second acoustic energy; areceive beamformer configured to receive electrical signals from theelements, to beamform samples from the electrical signals responsive tothe first beams, and to compress the electrical signals responsive tothe second beams using a Fourier transform applied across the elementsfor each time; and a processor configured to generate imaginginformation for the first imaging mode from beamformed samples and togenerate imaging information for the second imaging mode from thecompressed electrical signals.
 2. The system of claim 1 wherein thetransmissions of the first and second acoustic energy are interleaved inon-going scanning of a patient and wherein the imaging information forthe first and second imaging modes are combined in an image.
 3. Thesystem of claim 1 wherein the first imaging mode comprises flow mode andthe second imaging mode comprises B-mode.
 4. The system of claim 1wherein the processor is configured to reconstruct the electricalsignals from the elements responsive to the second beams from thecompressed electrical signals.
 5. The system of claim 4 wherein theprocessor is configured to reconstruct by applying an inverse Fouriertransform to the compressed electrical signals.
 6. The system of claim 1wherein the processor is configured to apply another Fourier transformas a function of time to the compressed electrical signals, interpolatein the frequency domain, and generate the imaging information for thesecond mode by two-dimensional inverse Fourier transformation.
 7. Thesystem of claim 1 wherein the receive beamformer comprises phaserotators and amplifiers in channels connected to each of the elements,and wherein the receive beamformer is configured to compress with aphase profile for geometric relationship of zero across the elements anda complex apodization function implemented by the phase rotators andamplifiers.
 8. The system of claim 1 wherein the receive beamformercomprises phase rotators or delays and amplifiers in channels connectedto each of the elements, and wherein the receive beamformer isconfigured to compress with a unity apodization function applied by theamplifiers and a linear delay profile applied by the phase rotators ordelays.
 9. The system of claim 8 wherein the linear delay profileapplied by the phase rotators or delays is a function of lateralposition of the element and a steering angle.
 10. The system of claim 8wherein the first imaging mode has a higher imaging frequency or agreater field of view than the second imaging mode.
 11. A method forultrasound beamformer-based channel data compression, the methodcomprising: receiving channel data from elements of a transducer array;encoding the channel data laterally across the array with a delay andsum beamformer with a set of basis functions, the basis functionssubstantially enabling recovery of the channel data from an output ofthe encoding; repeating the encoding for additional channel data; andtransmitting the output of the encoding.
 12. The method of claim 11wherein encoding the channel data laterally comprises encoding acrosschannels such that each channel contributes data in the encoding. 13.The method of claim 11 wherein encoding with the delay and sumbeamformer comprises encoding with an apodization profile being unityand a linear delay profile.
 14. The method of claim 11 wherein encodingwith the delay and sum beamformer comprises encoding with ageometry-based delay profile being zero and a complex apodizationprofile.
 15. The method of claim 11 wherein encoding with the basisfunctions comprises encoding with a Fourier transform implemented by thedelay and sum beamformer.
 16. The method of claim 11 wherein encodingwith the basis functions comprises encoding for lossless or lossyrecovery of the channel data.
 17. The method of claim 11 whereintransmitting comprises transmitting to a software-based imageprocessing, the software-based image processing including beamforming.18. The method of claim 17 wherein the beamforming comprises Fourierbeamforming, the Fourier beamforming comprising transforming the outputas a function of time, interpolating, and inverse two-dimensionalspatial Fourier transforming.
 19. The method of claim 11 furthercomprising: using the delay and sum beamformer to beamform other channeldata, the use of the delay and sum beamformer to beamform interleavedwith encoding with the delay and sum beamformer.
 20. A method forultrasound beamformer-based channel data compression, the methodcomprising: sampling a plurality of channels of element signals;transforming, by a receive beamformer, samples from a sampling domaininto spatial frequency data in a frequency domain; inverse transformingthe frequency data; and generating, by a processor, an ultrasound imagefrom an output of the inverse transforming.